Two other comrades, known only as Marine B and Marine C, were acquitted of murder by the court martial board in Bulford, Wiltshire.

The killing happened five months into an arduous six-month tour of Helmand province in 2011, known as Operation Herrick 14.

Blackman shot the Afghan, who had been seriously injured in an attack by an Apache helicopter, in the chest at close range with a 9mm pistol before quoting a phrase from Shakespeare as the man convulsed and died in front of him.

"There you are. Shuffle off this mortal coil, you c***. It's nothing you wouldn't do to us," Blackman told him.

Blackman then turned to comrades and said: "Obviously this doesn't go anywhere, fellas. I just broke the Geneva Convention."

The execution was filmed by a camera mounted on the helmet of Marine B.

Marines B and C were alleged to have been "party to the killing" and "encouraged and assisted" Marine A in committing the murder but they were cleared.

Blackman was given a life sentence with the minimum term decided by the seven-man court martial board made up of senior non-commissioned officers and officers, and Judge Advocate General Jeff Blackett.

During his evidence at the court martial, Blackman, who denied murder, admitted he fired his gun out of anger but insisted the insurgent was already dead.

He blamed "foolish bravado" for quoting Shakespeare at the dying man and said it was something "I am not proud of".

Sentencing the marine, the judge condemned his treatment of the Taliban insurgent and said he had put the lives of other British service personnel at risk.

"You were obliged to care for him but instead you executed him.

"You treated that Afghan man with contempt and murdered him in cold blood. By doing so you have betrayed your corps and all British service personnel who have served in Afghanistan, and you have tarnished their reputation.

"The victim was particularly vulnerable because he was seriously wounded and lying helpless and in obvious pain while you considered what to do with him.

"Your actions have put at risk the lives of other British service personnel. You have provided ammunition to the terrorists whose propaganda portrays the British presence in Afghanistan as part of a war on Islam in which civilians are arbitrarily killed.

"That ammunition will no doubt be used in their programme of radicalisation. That could seriously undermine the reputation of British forces and ultimately the mission in Afghanistan."